1. Field of the Invention
This invention relates to a method and apparatus for detecting cracks in a main load bearing spar of a helicopter rotor blade.
2. Description of the Prior Art
GB-A- 1218954 describes a crack detection system in which the interior of a hollow spar is pressurised and an indicator is provided to indicate any loss of pressure such as may be caused by a crack in the spar. This device requires special seals at the root and tip ends of the blade which increases the weight of the blade and can result in erroneous indications of a crack in the event of a small leak past a seal.
Furthermore, such a system cannot easily be adapted for use with modern composite blades in which a load bearing spar and trailing edge portion may be wrapped in outer layers of fibre-reinforced material which can preclude the escape of gas from a crack in the spar to provide an erroneous indication of structural integrity. GB-A-1361174 proposes one solution to this problem in that gas escape channels are provided along what are determined as the most vulnerable crack propagation areas in order to provide a gas escape route; however, such a system has practical lixitations.
GB-A-2108065 discloses a helicopter rotor blade to hub arm attachment incorporating an integrity monitoring system which includes a small annulus between a pre-loaded central bolt and an internal bore of the hub arm filled with a dye. The dye is pressurised by a bellows so as to leak through any crack occuring in the spindle to indicate a fault by visual inspection. Such a system is not applicable to the aforementioned problem of detecting cracks in a hollow load bearing rotor blade spar due to its large internal volume and the complex dynamic characteristics required during operation.
US-A-4135386 discloses a method and related apparatus for detecting cracks in porous material. The method comprises affixing a frangible fluid impermeabIe enclosure to the surface, creating a pressure differential between the interior and exterior of the enclosure and monitoring the internal pressure in the enclosure. A crack in the permeable material destroys the integrity of the enclosure, which in an embodiment consists of glass tube, and pressure sensor means are provided to detect a resulting pressure change. In preferred embodiments a vacuum is created within the enclosure. The requirement to provide monitoring and sensor means again precludes such a system for use in the detection of cracks in a hollow helicopter rotor blade spar.